1. Field of the Invention
The present invention generally relates to a method for producing a magnetic memory device. More specifically, the invention relates to a method for producing a magnetic random access memory (MRAM) having a magnetoresistive effect element in a memory cell portion.
2. Related Art
A magnetic random access memory (MRAM) is a memory unit which has magnetoresistive effect elements, exhibiting magnetoresistive effect in a cell portion for storing information and which is noticed as a next generation memory device characterized by a rapid operation, a large capacity and nonvolatility. The magnetoresistive effect is a phenomenon wherein electrical resistance varies in accordance with the direction of magnetization of a ferromagnetic material when a magnetic field is applied to the ferromagnetic material from the outside. A memory unit (MRAM) can be operated by using such a direction of magnetization of a ferromagnetic material for recording information and by reading information on the basis of the magnitude of an electrical resistance corresponding thereto.
In recent years, taking advantage of the fact that a rate of change in resistance due to the magnetoresistive effect (MR ratio) has reached 20% or more by the tunneling magnetoresistive effect (which will be also hereinafter referred to as the TMR effect) in a ferromagnetic tunnel junction including a sandwich structure wherein an insulating layer (a tunnel barrier layer) is sandwiched between two ferromagnetic layers, MRAMs using a tunneling magnetoresistive effect element (which will be also hereinafter referred to as a TMR element) utilizing the tunneling magnetic effect are expected and noticed.
In use of a TMR element for a magnetoresistive effect element of a memory cell of an MARM, a magnetization fixed layer, in which the direction of magnetization is fixed to one direction so as not to vary, one of two ferromagnetic layers sandwiching a tunnel barrier layer therebetween, is used as a magnetization reference layer whereas a magnetization free layer, in which the direction of magnetization can be easily inverted to another direction, the other layer of the two ferromagnetic layers is used as a storage layer. Information can be stored by causing states, in which the directions of magnetization in the reference layer and storage layer are parallel and antiparallel to each other, to correspond to binary information “0” and “1”.
Information to be recorded is written by inverting the direction of magnetization in the storage layer by a magnetic field induced by a current flowing through a writing line provided in the vicinity of a TMR element. The recorded information is read by detecting a variation of resistance due to the TMR effect. Therefore, in the storage layer, the MR ratio due to the TMR effect is preferably large, and the magnetic field required to invert magnetization, i.e., the switching magnetic field, is preferably small.
On the other hand, it is required to fix the direction of magnetization in the reference layer so as to be difficult to be inverted. Therefore, there is used a structure wherein an antiferromagnetic layer is provided so as to contact a ferromagnetic layer serving as the reference layer so that it is difficult to invert magnetization by the exchange coupling force. Such a structure is called a spin-valve structure. In this structure, the direction of magnetization in the reference layer is determined by annealing while applying a magnetic field (a magnetization fixing annealing).
By the way, since the magnetization inversion in the storage layer uses the magnetic field induced by the current flowing through the writing line as described above, there is a problem in that, if the switching magnetic field in the storage layer is large, the current flowing through the writing line increases, so that electric power consumption increases. In order to solve this problem, as shown in FIG. 12A, there is provided a bit line 40 with yoke wherein a writing bit line 40, which is one of a writing word line 10 and writing bit line 40 serving as writing lines, is covered with a yoke 80 of a soft magnetic material to intensify a current induced magnetic field, which is produced from the writing bit line 40, in the vicinity of the TMR element 20. In such a bit line 40 with yoke, as a distance between the tip end portion of the yoke 80 and the storage layer 206 decreases, the magnetic field produced in the vicinity of a storage layer 206 is stronger, so that magnetization in the storage layer can be inverted by a smaller writing current. Furthermore, in FIG. 12A, reference number 75 denotes an insulating layer.
Therefore, as shown in FIG. 12B, the tip end portion of the yoke 80 is preferably extended below the writing bit line 40 so as to approach the TMR element 20.
By the way, when the writing bit line 40 is formed, there is some possibility that the writing bit line 40 is displaced from the TMR element 20. Therefore, the width of the writing bit line 40 is usually designed to be larger than the TMR element 20 by making allowance for the displacement.
In addition, in the structure where the yoke 80 is extended to the vicinity of the TMR element 20 as shown in FIG. 12B, there is some possibility that the yoke 80 contacts the TMR element 20 to electrically short-circuit, so that it is required to increase the width of the writing bit line 40 in view of a margin corresponding thereto. For example, if the displacement in alignment is 50 nm and the margin for preventing short-circuit is 50 nm, the width of the writing bit line 40 may be designed to be larger than the TMR element 20 by 100 nm on both sides thereof. As a result, the distance between the storage layer 206 of the TMR element 20 and the tip end of the yoke 80 is at least 100 nm, and this distance is increased to 150 nm at the maximum if the writing bit line 40 is displaced. That is, there are problems in that the distance between the storage layer 206 and the yoke 80 increases and varies, so that the switching current itself passing through the writing bit line 40 can not be decreased, whereby the variation in magnitude of current required for switching is large.